Zirconia demonstrates very high refractoriness but its use in bodies of any substantial bulk has been severely limited because of an extremely disruptive, reversible phase transformation which takes place in the vicinity of 1000.degree.-1100.degree. C.; viz., the polymorphic conversion of the tetragonal crystal form to the monoclinic form. Hence, thermal cycling of ZrO.sub.2 bodies through the transformation range, about 900.degree.-1200.degree. C., commonly results in cracking and, not infrequently, the total disintegration thereof, because of the relatively large volume change which accompanies that conversion of crystal forms. Consequently, extensive investigations have been conducted to develop "alloys" of ZrO.sub.2 with another metal oxide, the most notable of such being CaO, MgO, and Y.sub.2 O.sub.3.
The initial research efforts sought to produce a stabilized cubic zirconia (the high temperature ZrO.sub.2 structure) and the recited "alloying" oxides are especially effective in forming stable solid solutions with ZrO.sub.2 having the cubic fluorite structure. It was soon observed, however, that fully stabilized cubic ZrO.sub.2 bodies are not particularly strong or resistant to thermal shock. Continued investigations have demonstrated that partially stabilized ZrO.sub.2 bodies can be both stronger and more thermal shock resistant than either unstabilized or completely stabilized ZrO.sub.2 articles. The following patents are illustrative of the considerable effort that has been expended in devising ZrO.sub.2 bodies manifesting high refractoriness coupled with high strength and good resistance to thermal shock.
U.S. Pat. No. 3,365,317 was directed to the production of ZrO.sub.2 bodies illustrating the properties of lubricity, toughness, abrasion resistance, and chemical inertness, thereby recommending their utility as drawing die materials. The bodies consisted essentially, in weight percent, of 96.5-97.2% ZrO.sub.2 and 2.8-3.5% MgO, and exhibited coefficients of thermal expansion [room temperature (R.T.) to 1400.degree. C.] not greater than 73.times.10.sup.-7 /.degree.C., compressive strengths of at least 250,000 psi, and moduli of rupture of at least 27,000 psi. The bodies were sintered at temperatures of 2500.degree.-3500.degree. F. (.about.1371.degree.-1927.degree. C.) and the fired product contained tetragonal ZrO.sub.2 and cubic and/or monoclinic ZrO.sub.2. No quantitative measurement of the proportion of each crystal phase of ZrO.sub.2 present was provided, but the objective was to optimize the amounts of tetragonal and monoclinic ZrO.sub.2 in the bodies. It was emphasized that impact strength and abrasion resistance decreased with MgO levels greater than 3.5%.
U.S. Pat. No. 3,620,781 was concerned with a method for producing bodies of ZrO.sub.2 which were partially stabilized through the inclusion of 2-5% by weight CaO. The bodies were characterized by high moduli of rupture and high Young's moduli. The microstructure of the bodies consisted essentially of cubic ZrO.sub.2 grains as the major phase by volume, intergranular primary monoclinic ZrO.sub.2 grains of substantially smaller average grain size than that of the cubic grains, and an extremely fine-grained precipitate of monoclinic ZrO.sub.2 dispersed intragranularly throughout the cubic grains. The precipitate had an average grain size much smaller (at least one order of magnitude) than the average grain size of the primary monoclinic ZrO.sub.2 grains, viz., about 0.1-2 microns in comparison to 10-20 microns.
The inventive method comprised firing the bodies at a temperature of at least 1800.degree. C. and up to the melting temperature of the body, annealing the sintered body by holding it within the range of 900.degree.-1700.degree. C. for a substantial period of time (desirably for at least one day), and then cooling to ambient temperature. The annealing period produces the required precipitate which, in turn, imparts the enhanced strength and elastic modulus to the bodies.
U.S. Pat. No. 3,634,113 described the production of cubic ZrO.sub.2 bodies stabilized with 6-20 mole percent of a type C mixed rare earth oxide solid solution containing Yb.sub.2 O.sub.3, Er.sub.2 O.sub.3, Dy.sub.2 O.sub.3, Ho.sub.2 O.sub.3, Tb.sub.2 O.sub.3, Tm.sub.2 O.sub.3, and Lu.sub.2 O.sub.3. It was asserted that the final bodies were totally free from monoclinic ZrO.sub.2 and might consist solely of cubic ZrO.sub.2 or might contain a small quantity of tetragonal ZrO.sub.2 with the cubic polymorph.
U.S. Pat. No. 3,887,387 claimed a method for preparing ZrO.sub.2 bodies by sintering a mixture of 30-90% pulverulent monoclinic ZrO.sub.2, 7.8-69.5% pulverulent ZrO.sub.2 stabilized with at least one stabilizing oxide selected from the group of MgO, CaO, CdO, MnO.sub.2, CoO, TiO.sub.2, and rare earth oxides, and 0.5-2.2% of at least one pulverulent stabilizing oxide of the group recited above. The total amount of stabilizing oxide ranged between 2.5-3.5%. The final product was asserted to consist of 75-95% cubic crystals.
U.S. Pat. No. 4,035,191 disclosed means for producing articles of stabilized ZrO.sub.2 having microstructures containing less than 10% cubic ZrO.sub.2 and at least 5% tetragonal ZrO.sub.2 which do not destabilize at temperatures between 1650.degree.-2250.degree. F. (.about.899.degree.-1232.degree. C.). The bodies consisted essentially, in weight percent, of 0.1-5.0% ZnO, at least 0.5% of a primary stabilizer selected from the group of 0.25-4% MgO, 0.25-4% Y.sub.2 O.sub.3, and mixtures thereof, and the balance ZrO.sub.2. The bodies are sintered between about 2550.degree.-2950.degree. F. (.about.1399.degree.-1621.degree. C.).
U.S. Pat. No. 4,067,745 was drawn to a process for forming a partially stabilized ZrO.sub.2 body which consisted of firing, between 1700.degree.-1950.degree. C., a body consisting essentially of 3.3-4.7% by weight CaO and the remainder ZrO.sub.2, cooling the body at an average rate of at least 175.degree. C./hour to a temperature between 1200.degree.-1450.degree. C., and aging the body for a period of time (typically about 64 hours) within that latter temperature range. The microstructure of the fired body was stated to be composed of metastable tetragonal domains of critical size within cubic matrix grains. It was asserted that the same controlled microstructure could be developed in MgO-stabilized ZrO.sub.2 bodies, but those products are stated to be subject to two problems:
(a) the reaction kinetics are so rapid that it is difficult to introduce quality control procedures into the manufacturing process; and
(b) the MgO-ZrO.sub.2 compositions are subject to a eutectoid decomposition reaction below 1400.degree. C.
U.S. Pat. No. 4,279,655 was directed to a method for preparing bodies of ZrO.sub.2 which are partially stabilized with MgO. The method claimed comprised the steps of:
(1) mixing and wet milling 2.8-4.0% by weight MgO, balance ZrO.sub.2, to a mean particle size of 0.7 micron, the ZrO.sub.2 containing no more than 0.03% SiO.sub.2 ;
(2) calcining the powdered material at 800.degree.-1450.degree. C. for about 24 hours;
(3) wet milling the mixture to a mean particle size of 0.7 micron;
(4) molding the wet mixture into a desired shape;
(5) firing the shape at 1550.degree.-1800.degree. C.;
(6) cooling the shape to a temperature between 800.degree. C. and ambient temperature to induce nucleation, the rate of cooling being controlled so that a tetragonal ZrO.sub.2 precipitate phase forms in the sintered material and coarsens to an elliptical precipitate having a major axis of about 1500 .ANG.;
(7) heating the body to an aging/transformation range of 1000.degree.-1400.degree. C.;
(8) holding the body within that temperature range until 2-30% of the tetragonal ZrO.sub.2 precipitate is transformed into monoclinic ZrO.sub.2 material; and
(9) cooling to room temperature.
The microstructure of the final body was described as being composed predominantly of relatively large cubic grains within which grains are elliptically-shaped precipitates of tetragonal ZrO.sub.2 having a long dimension of about 1500 .ANG.. The precipitate comprised 2-10% of the material. A grain boundary phase of monoclinic ZrO.sub.2 was also present constituting 8-15% of the material. Finally, a monoclinic phase, formed via the transformation of some of the tetragonal precipitate, was dispersed within each cubic ZrO.sub.2 grain; that monoclinic phase comprising about 0.5-20% of the material. The monoclinic grains were asserted to be untwinned, in contrast to the twinning conventionally observed in those precipitates when they are transformed from the tetragonal state. The aging step was posited as producing the untwinned monoclinic crystals.
As can be observed from the above review of patent literature, recent research in the field of ZrO.sub.2 stabilization has been directed to the formation of partially stabilized ZrO.sub.2, the means for accomplishing that goal generally involving two mechanisms. The first has utilized compositional variations; e.g., the use of two or more stabilizing oxides rather than a single stabilizer. To illustrate, when a mixture of CaO+MgO was employed as the stabilizer, it was discovered that the rate of destabilization of ZrO.sub.2 above 1000.degree. C. is slower than for either stabilizer alone. The second mechanism has commonly employed a single stabilizer, accompanied with a special heat treatment schedule.
This latter mechanism, which has been termed transformation toughening, has been studied extensively utilizing either CaO or MgO as the stabilizer. MgO has appeared to constitute the preferred stabilizer when using the fine intragranular precipitate approach to toughening. Nevertheless, as has been observed above, that method is very difficult to control. The aging time and the temperature in the cooling and reheating steps must be regulated with extreme care to induce the proper amount of stabilizer movement necessary to produce the desired precipitate of tetragonal ZrO.sub.2 of the correct size within the grains of cubic ZrO.sub.2. That phenomenon takes place within a range of temperatures wherein the MgO tends to exsolve from the solid solution given sufficient time, leaving a two-phase structure of MgO and monoclinic ZrO.sub.2 at room temperature.